1. Field of the Invention
The present invention relates to a liquid crystal display in which a liquid crystal layer containing a polymerizable component (monomer or oligomer), which is polymerized by light or heat, is sealed between substrates, and the polymerizable component is polymerized while a voltage is applied to the liquid crystal layer to fix a tilting direction of a liquid crystal molecules, and a method of manufacturing the same.
Besides, the present invention relates to a liquid crystal display of a VA (Vertically Aligned) mode in which a liquid crystal having a negative dielectric anisotropy is vertically aligned, and a method of manufacturing the same.
2. Description of the Related Art
A multi-domain vertical alignment mode liquid crystal display (hereinafter abbreviated to an MVA-LCD) is known in which a liquid crystal having a negative dielectric anisotropy is vertically aligned and a bank (linear protrusion) or a cut portion (slit) of an electrode is provided on the substrate as an alignment regulating structural member. Since the alignment regulating structural member is provided, even if a rubbing processing is not performed to an alignment film, liquid crystal alignment orientations at the time of voltage application can be controlled to be plural orientations. This MVA-LCD is superior to a conventional TN (Twisted Nematic) LCD in a visual angle property.
However, the conventional MVA-LCD has a defect that white luminance is low and a display is dark. The main cause of this is that since an upper portion of a protrusion or an upper portion of a slit becomes a boundary of alignment division to generate a dark line, the transmissivity at the time of a white display becomes low and the display becomes dark. In order to improve this defect, it is sufficient if an arrangement interval of the protrusions or slits is made sufficiently wide. However, since the number of the protrusions or slits as the alignment regulating structural members becomes small, there arises a problem that it takes a time to fix the alignment of LC molecule even if a predetermined voltage is applied to the liquid crystal, and a response speed becomes low.
In order to solve this problem and to obtain an MVA-LCD which has high luminance and enables a high speed response, a polymer fixation (macromolecule fixation) system is effective. In the polymer fixation system, a liquid crystal composite in which a polymerizable component of a monomer, an oligomer, or the like (hereinafter abbreviated to a monomer) is mixed in a liquid crystal, is sealed between substrates. In the state where liquid crystal molecules are tilted by applying a voltage between the substrates, monomers are polymerized into polymers. By this, a liquid crystal layer in which the molecules are tilted (inclined) at a predetermined tilt direction by voltage application is obtained, and tilting direction of the liquid crystal molecule can be fixed. A material which is polymerized by heat or light (ultraviolet ray) is selected as the monomer.
However, the polymer fixation system has some problems relating to unevenness of display when an image is displayed on a completed LCD. First, there is a problem that unevenness of display occurs on an image display of the completed LCD due to the alignment abnormality of liquid crystal locally generated in driving the liquid crystal at the time of monomer polymerization. Besides, there is also a problem that there occurs unevenness of display due to the abnormality of characteristics of thin film transistors (TFTs) caused by driving of liquid crystal and polymerization processing at the time of monomer polymerization.
FIG. 21A shows a liquid crystal driving method at the time of forming a polymer (polymerization) in a conventional MVA-LCD to which an alignment fixation processing by the polymer fixation system is performed. FIG. 21B shows the cause of the unevenness of display of the MVA-LCD in which the polymer formed by the liquid crystal driving method shown in FIG. 21A exists in a liquid crystal layer. The n-channel type TFTs are used in this MVA-LCD.
In general, in order to prevent a ghosting phenomenon, an alternating voltage is applied to a liquid crystal layer of an LCD. Then, also in a polymerization step at a stage of LCD manufacture, an alternating voltage is applied to the liquid crystal layer to tilt the liquid crystal molecules, and monomers are polymerized. For example, as shown in a graph of FIG. 21A, a gate voltage Vg=33 V is kept applied to all gate bus lines of a panel display region, and a TFT, which is provided in each pixel, is kept in an on state, and then, a drain voltage in which an alternating data voltage Vd (ac)=±7 V is superimposed on a direct-current data voltage Vd (dc)=13 V is applied to all drain (data) bus lines. By this, Vd (dc)+Vd (ac) is written to a pixel electrode formed in each pixel region. On the other hand, a common electrode arranged opposite to the pixel electrode across the liquid crystal layer is kept at a common voltage Vc=13 V. By this, the alternating voltage of the data voltage Vd (ac)=±7 V is applied to the liquid crystal layer.
FIG. 21B shows the unevenness of display of the MVA-LCD fabricated by this liquid crystal driving method. FIG. 21B shows a display state of three pixels arranged in order of G (Green), B (Blue) and R (Red) from the left. A dark portion X1 and a bright portion X2 shown in a vertical ellipse in the drawing are seen. It is understood that as stated above, if polymer fixation is performed by the driving method shown in the graph of FIG. 21A, the alignment of the liquid crystal in the pixel, especially the alignment state in the vicinity of a pixel edge fluctuates and the dark portion X1 is formed as shown in FIG. 21B. Besides, there arises a problem that when the whole display region of the panel in the state like this is observed, the display is seen to be rough.
Besides, in the above liquid crystal driving method, the gate voltage Vg is made sufficiently larger than the voltage Vd (dc)+Vd (ac) of the drain bus line to turn on the TFT, and then, the voltage Vd (dc)+Vd (ac) for tilting the liquid crystal molecules is applied to the drain bus line. However, if polymerization is made in this driving state, a large fluctuation occurs in threshold values of the respective TFTs provided in the respective pixels, and there arises a defect that a desired display can not be produced or the unevenness of display occurs since some TFT is not turned on in a portion on the display region of the completed LCD.
Besides, there is a case where an alignment regulating structural member is provided to keep the liquid crystal in a desired alignment orientation at the time of monomer polymerization. As the alignment regulating structural member, there is, for example, a structure used in a subsequent embodiment and shown in FIG. 4A. In this structure, linear cruciform connection electrodes 12 and 14 dividing a rectangular pixel into four rectangles of the same shape are formed. The connection electrode 12 is formed at the substantially center portion of the rectangular pixel and parallel with a long side, and the connection electrode 14 is formed on a storage capacitance bus line 18 crossing the substantially center portion in the pixel.
A plurality of stripe-like electrodes 8 of a minute electrode pattern are formed to be repeatedly extended from the connection electrodes 12 and 14 at an angle of 45°. A pixel electrode is constituted by the connection electrodes 12 and 14 and the plurality of stripe-like electrodes 8. A space 10 in a state in which a portion of an electrode is cut away is formed between the adjacent stripe-like electrodes 8. The stripe-like electrode 8 and the space 10 constitute an alignment regulating structural member. Incidentally, instead of the stripe-like electrode 8 and the space 10 of FIG. 4A, a minute linear protrusion may be naturally formed on a pixel electrode formed on the whole surface in a pixel.
When such a minute line and space pattern is formed, liquid crystal molecules are aligned in parallel with the longitudinal direction of the minute pattern. By doing so, alignment division boundary portions in the pixel can be made as small as possible. However, there arises a problem that T-V characteristics (transmissivity-gradation voltage characteristics) are changed by slight fluctuation of the width of the minute electrode pattern due to fluctuation of an exposure pattern in a photolithography process, and this is seen as the unevenness of display.
Besides, as described above, since a rubbing processing is not performed to the alignment film in the MVA-LCD, means for regulating the alignment orientation with respect to liquid crystal molecules in the outside region of the pixel electrode is not provided. Thus, as shown in FIG. 20A, there is a case where singular points (indicated by ∘ or • in the drawing) of alignment vectors are generated outside the pixel electrode at random, and the alignment is maintained as it is. Thus, if monomers are polymerized in a state where liquid crystal molecules 24a outside the pixel electrode or in the vicinity of an edge of the pixel electrode are aligned in an orientation other than a desired one, as shown in FIG. 20A, a dark line is formed in a region connecting the adjacent singular points, and there arises a problem that the luminance is lowered, a response time becomes long, or the unevenness of display occurs.